Power
Power is energy flowing! It is measured as energy per unit of time
- Energy is in Joules (J)
- Power is in Joules per second (J/s) which has the special unit of Watts (W)
1 Watt = 1 Joule of energy per second
1 W = 1 J/s
Power Formulas
No, not that type of Power Formula!
Power is the rate of energy per time:
P = \frac{Energy}{Time }
Example: 3000 J of energy is used in 20 seconds, what is the power?
P = \frac{3000 J}{20 s } = 150 W
We can also change the formula this way:
P = \frac{Energy}{Time } | ||
Energy is also Force × Distance, so: | P = \frac{Force × Distance}{Time } | |
Distance/Time is Velocity, so: | P = Force × Velocity |
Example: These guys pull with a combined force of 4000 N, and manage a velocity of 0.4 m/s, what power are they using?
P = 4000 N x 0.4 m/s
= 1600 N m/s
= 1600 W
Hang on! Is N m/s (Newton meter per second) really the same as W (Watt)?
In fact all these are the same:
Power is: | ||
Start with: | N m/s | |
A Newton is kg m/s^{2}: | kg m/s^{2} m/s | |
Simplify: | kg m^{2}/s^{3} | |
A Joule is kg m^{2}/s^{2}: | J/s | |
A Watt is J/s: | W |
Horsepower
1 horsepower (hp) is 745.7 W (or 746 W for electrical motors)
Example: Sam's car has a 200 hp engine. What is that in watts?
200 × 745.7 = 149,140 W = 149 kW (to nearest kW)
What is a kiloWatt Hour?
1 kWh is the Energy used when the Power is 1 kW for 1 hour.
This is a little 1 kW radiator. How much energy does it use in an hour?
- 1 kW is 1000 Watts which is 1000 J/s
- There are 3600 seconds in an hour
So in an hour it uses:
1000 J/s × 3600 s = 3,600,000 J = 3.6 MJ
And so 1 kWh (kiloWatt hour) is 3.6 MJ of Energy